You have decided to go camping with some friends. While out on a hike, you reach the top of a ridge and look down at the trail behind you. In the distance, you can see your camp. You're thinking about how far you've traveled, and wonder if there is a way to determine it.

By using a small device called a clinometer, you're able to measure the angle between your horizontal line of sight and the camp as
, and you know that the hill you just hiked up has a height of 300 m. Is it possible to find out how far away your camp is using this information? (Assume that the trail you hiked is slanted like the side of a triangle.)

Keep reading and at the conclusion of this Concept, you'll know how to solve this problem.

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Guidance

You can use right triangles to find distances, if you know an angle of elevation or an angle of depression. The figure below shows each of these kinds of angles.

The angle of elevation is the angle between the horizontal line of sight and the line of sight up to an object. For example, if you are standing on the ground looking up at the top of a mountain, you could measure the angle of elevation. The angle of depression is the angle between the horizontal line of sight and the line of sight
down to
an object. For example, if you were standing on top of a hill or a building, looking down at an object, you could measure the angle of depression. You can measure these angles using a clinometer or a theodolite. People tend to use clinometers or theodolites to measure the height of trees and other tall objects. Here we will solve several problems involving these angles and distances.

Example A

You are standing 20 feet away from a tree, and you measure the angle of elevation to be
. How tall is the tree?

Solution:

The solution depends on your height, as you measure the angle of elevation from your line of sight. Assume that you are 5 feet tall.

The figure shows us that once we find the value of
, we have to add 5 feet to this value to find the total height of the triangle. To find
, we should use the tangent value:

Example B

You are standing on top of a building, looking at a park in the distance. The angle of depression is
. If the building you are standing on is 100 feet tall, how far away is the park? Does your height matter?

Solution:

If we ignore the height of the person, we solve the following triangle:

Given the angle of depression is
,
in the figure above is
. We can use the tangent function to find the distance from the building to the park:

If we take into account the height if the person, this will change the value of the adjacent side. For example, if the person is 5 feet tall, we have a different triangle:

If you are only looking to estimate a distance, then you can ignore the height of the person taking the measurements. However, the height of the person will matter more in situations where the distances or lengths involved are smaller. For example, the height of the person will influence the result more in the tree height problem than in the building problem, as the tree is closer in height to the person than the building is.

Example C

You are on a long trip through the desert. In the distance you can see mountains, and a quick measurement tells you that the angle between the mountaintop and the ground is
. From your studies, you know that one way to define a mountain is as a pile of land having a height of at least 2,500 meters. If you assume the mountain is the minimum possible height, how far are you away from the center of the mountain?

Solution:

Guided Practice

1. You are six feet tall and measure the angle between the horizontal and a bird in the sky to be
. You can see that the shadow of the bird is directly beneath the bird, and 200 feet away from you on the ground. How high is the bird in the sky?

2. While out swimming one day you spot a coin at the bottom of the pool. The pool is ten feet deep, and the angle between the top of the water and the coin is
. How far away is the coin from you along the bottom of the pool?

3. You are hiking and come to a cliff at the edge of a ravine. In the distance you can see your campsite at the base of the cliff, on the other side of the ravine. You know that the distance across the ravine is 500 meters, and the angle between your horizontal line of sight and your campsite is
. How high is the cliff? (Assume you are five feet tall.)

Solutions:

1. We can use the tangent function to find out how high the bird is in the sky:

We then need to add your height to the solution for the triangle. Since you are six feet tall, the total height of the bird in the sky is 173.8 feet.

2. Since the distance along the bottom of the pool to the coin is the same as the distance along the top of the pool to the coin, we can use the tangent function to solve for the distance to the coin:

3. Using the information given, we can construct a solution:

This is the total height from the bottom of the ravine to your horizontal line of sight. Therefore, to get the height of the ravine, you should take away five feet for your height, which gives an answer of 228 meters.

Concept Problem Solution

Since you know the angle of depression is
, you can use this information, along with the height of the hill, to create a trigonometric relationship:

Since the unknown side of the triangle is the hypotenuse, and you know the opposite side, you should use the sine relationship to solve the problem:

You have traveled approximately 498.5 meters up the hill.

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. How tall is the tree? Assume you are 5 feet tall up to your eyes.

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and the depth of the ocean, at that point is 350 feet. How far away is she from the reef?

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. If the building is 78 ft tall, how far away is the car?

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. If the building is 78 ft tall, how far away is the fountain?

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. How tall is the tree? Assume you are 5 feet tall up to your eyes.

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. How wide is the river?

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Eric is flying his kite one afternoon and notices that he has let out the entire 100 ft of string. The angle his string makes with the ground is
. How high is his kite at this time?

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angle with the ground 25 ft from the base of the tree. What was the height of the tree to the nearest foot?

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. To the nearest mile, find the ground distance from the airplane to the tower.

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Description

Find distance using right triangles and angles of elevation or depression

Learning Objectives

Here you'll learn how to use a triangle to find a distance if you know the angle of elevation or depression.